1. Field of the Invention
The present invention relates to an epitaxial wafer having a gallium nitride (GaN) epitaxial layer deposited on a semiconductor substrate and the method for preparing the epitaxial wafer, which is used for blue and green optoelectronic devices.
2. Description of Related Art
Referring to FIG. 1, a conventional gallium nitride blue light emitting device will be explained. FIG. 1 shows a sectional view of a conventional GaN blue light emitting device. The GaN blue light emitting device comprises an epitaxial wafer including a sapphire substrate 1, a GaN buffer layer 2 deposited on the sapphire substrate 1, and a hexagonal GaN epitaxial layer 3, a first cladding layer 4, a luminescence layer 5, a second cladding layer 6 and a GaN epitaxial layer 7 stacked in the named order on the epitaxial wafer. Ohmic electrodes 8 and 9 are respectively arranged on the GaN epitaxial layers 3 and 7. The GaN buffer layer 2 is disposed to relax distortion caused by difference in lattice parameters between the sapphire substrate 1 and GaN epitaxial layer 3.
Since the conventional light emitting device shown in FIG. 1 uses an insulating sapphire substrate 1, necessary two ohmic electrodes 8 and 9 should be formed at the same side of the substrate 1. This requires at least two times of patterning process by using photolithography. It is also necessary to etch nitride by reactive ion etching, which complicates the manufacturing process of the device. In addition, sapphire has high hardness which make it difficult to divide the devices into individual ones.
In prior arts, it has been tried to use a conductive gallium arsenic (GaAs) substrate instead of the sapphire substrate. For example, Okumura et al. grew cubic GaN on a (100) plane of a GaAs substrate (Journal of Crystal Growth, 164 (1996), pp. 149-153). However, cubic GaN grown on a (100) plane of a GaAs substrate generally has poor quality due to large amount of stacking fault, as shown in a transmission electron microscopy photograph of Okumura et al. This is considered to be caused by instability of cubic GaN which is of higher degree than hexagonal GaN.
On the other hand, it has been also tried to grow more stable hexagonal GaN on a (111) plane of a GaAs substrate. C. H. HONG et al. reported that hexagonal GaN was grown on a (111) A-plane and a (111) B-plane of GaAs substrate by metalorganic vapor phase epitaxy (MOVPE) (Journal of Electronic Materials, Vol. 24, No. 4, 1995, pp. 213-218). However, the grown hexagonal GaN had insufficient properties for use of a blue light emitting device. This is due to low growth temperatures of 800.degree. C. at the highest of the GaN epitaxial layer of C. H. HONG et al., contrary to the GaN layer of the blue light emitting device fabricated on a sapphire substrate grown by MOVPE at the growth temperature of higher than 1000.degree. C. C. H. HONG et al. grew the GaN epitaxial layer at a low temperature since arsenic having a high vapor pressure would escape from GaAs substrate at a temperature of around 600.degree. C.
As mentioned above, in prior art, when hexagonal GaN is epitaxially grown on a GaAs (111) substrate, the substrate temperature is raised to 850.degree. C. or so in order to prevent damage of the GaAs substrate caused by heating. By this, the hexagonal GaN epitaxial film obtained by the conventional MOVPE has carrier density of as high as 1.times.10.sup.19 cm.sup.-3 at non-dope, which is not suitable for a blue light emitting device.